This invention concerns filter units of a type including a tank adapted to receive the liquid to be filtered and a filter media belt driven through the tank over a perforated plate. A vacuum box beneath the perforated plate induces liquid flow through the filter media belt to be filtered, the liquid collected in the vacuum box. Such filter units are in wide use in connection with the filtration of machine tool liquid coolant, in which cutting chips and other solid debris are filtered from the coolant prior to being recirculated back to the machine too.
Such filter units have utilized both a disposable media belt and a permanent or recleanable filter media belt. An example of a disposable media belt filter is shown in U.S. Pat. No. 4,774,010 issued on Sep. 27, 1988 for a "Tank Filter With Conveyor Flight Driven Filter Media Belt"; U.S. Pat. No. 4,396,505 issued on Aug. 2, 1983 for a "Filtration System Pump Arrangement"; and U.S. Pat. No. 5,089,143 issued on Feb. 18, 1992 for a "Method of Filtering Industrial Liquids and Apparatus Therefor".
In filters of this type, drag conveyor chain loops on either side of conveyor flights engage the edges of the disposable filter media belt to seal and advance the same through its path within the filter tank. The disposable media is introduced at one end of the filter tank, driven by the conveyor chain loops over the vacuum box to the discharge end of the tank, where it is collected for disposal.
There has heretofore also been developed filter units of this type having a "permanent" or recleanable filter media belt, which is recirculated after passing through the filter tank. See for example U.S. Pat. No. 4,514,301 issued on Apr. 30, 1985, for a "Continuous Media Filter". The arrangement shown in that patent comprises a permanent filter media belt which is attached to the drag conveyor chain to be recirculated therewith. The attachment between the drag conveyor chain and the permanent filter belt makes it more difficult to replace the filter media belt, increases the initial cost of the filter media belt, and makes the cleaning of the permanent filter media belt more difficult inasmuch as the drag conveyor includes flight bars connected to the conveyor chain loops and extending across and over the filter media belt to impede access thereto.
In copending application Ser. No. 07/813,161, filed Dec. 24, 1991, for a "Belt Filter" there is described an arrangement for a recirculation of a permanent filter media belt driven only by a frictional engagement with the drag conveyor chain loops by recirculation of the filter media belt beneath the filter tank. This leaves the outer surface of the filter media belt accessible for thorough cleaning thereof during its recirculation and also allow the optional use of the disposable filter media by introduction between the permanent filter media belt and the chain loops of the conveyor.
See also U.S. Pat. No. 5,118,420 issued on Jun. 2, 1992, for "Self Cleaning Vacuum Filter Particularly For Filtering Industrial Lubricants and coolants" in which a permanent filter media belt is recirculated below the vacuum box but within the filter tank.
In recent years, the use of disposable filter media has become less desirable as the used disposable filter media may be loaded with entrapped contaminants which must be disposed of using special precautions and may not be able to be deposited in an open land fill. This significantly increases the costs of operation.
Accordingly, the use of permanent filter media has become more attractive to users of this equipment.
Inasmuch as these tank-type filter units are often placed below grade in pits, the retrofitting of disposable filter media filter units would be impractical where the permanent filter media belt is recirculated beneath the tank as described in the aforementioned copending U.S. patent application, and would add undue complexity to the equipment.
Accordingly, it is the object of the present invention to provide an arrangement and method for retrofitting tank filters with permanent filter media belts without increasing the complexity of the equipment or incurring excessive cost in the retrofit.